The present invention relates to an engine exhaust purification device provided with a catalyst.
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein an oxygen amount stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is estimated based on an engine intake air amount and an air fuel ratio of an exhaust flowing into the catalyst, and engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is constant.
To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of the three-way catalyst at a maximum, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. If the oxygen storage amount of the catalyst is maintained constant, oxygen in the exhaust is stored in the catalyst even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes lean, and conversely, oxygen stored in the catalyst is released even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes rich, so the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.
Therefore, in an exhaust purification device performing this type of control, it is required to calculate the oxygen storage amount precisely to maintain the conversion efficiency of the catalyst at a high level, and various methods of computing the oxygen storage amount have been proposed.
However, as the maximum oxygen storage amount slightly decreases due to catalyst deterioration, the target amount undergoes a relative shift from a suitable value and the conversion efficiency of the catalyst decreases, i.e., there is a risk that the exhaust performance will fall with time. To determine the catalyst deterioration, oxygen sensors may be installed upstream and downstream of the catalyst, and the deterioration determined by comparing the number of times their outputs invert, or alternatively, the difference of the maximum value and minimum value of the output each time the output of the downstream oxygen sensor invert during a predetermined number of times that the output of the upstream oxygen sensor inverts, may be computed, and deterioration determined when its average value is greater than a reference value.
In the above air-fuel ratio control using the three-way catalyst, if an A/F sensor (linear oxygen sensor) having linear characteristic according to the air-fuel ratio upstream is provided upstream of the catalyst to precisely determine the oxygen storage amount, as the amplitude of the output of the A/F sensor is small, and the number of inversions of the downstream oxygen sensor decreases the more stably air-fuel ratio control is performed, the catalyst deterioration determining frequency is smaller in the above prior art, and in the worst case, deterioration may not be determined at all.
It is therefore an object of this invention, which was conceived in view of the above problems, to provide an exhaust purification device permitting determination of catalyst deterioration without depending on the output inversion of the exhaust oxygen sensor.
In order to achieve above object, this invention provides an engine exhaust purification device comprises a catalyst provided in an engine exhaust passage, a sensor which detects an exhaust characteristic flowing into the catalyst, and a microprocessor programmed to compute an oxygen storage amount of the catalyst using the detected exhaust characteristic, to compute a target air-fuel ratio of the engine based on the computed oxygen storage amount such that the oxygen storage amount of the catalyst is a predetermined target value, and to determine a deterioration of the catalyst based on an integrated value of the oxygen storage amount for a predetermined time.
Further this invention provides an engine exhaust purification device comprises, a catalyst provided in an engine exhaust passage, a first sensor which detects an exhaust characteristic of flowing into the catalyst, a second sensor which detects an air-fuel ratio of exhaust flowing out of the catalyst, and a microprocessor programmed to compute the oxygen storage amount of the catalyst using the detected exhaust characteristic, to perform reset processing which initializes the oxygen storage amount to a maximum value when the air-fuel ratio of the exhaust from the catalyst detected via the second sensor exceeds a lean determining value, and initializes the oxygen storage amount to a minimum value when the air-fuel ratio of the exhaust from the catalyst detected via the second sensor exceeds a rich determining value, to compute a target air-fuel ratio of an engine so that the oxygen storage amount of the catalyst is a predetermined target value based on the computed oxygen storage amount, and to compare a reset processing frequency with a determining value, and determine that the catalyst has deteriorated when the reset processing frequency exceeds the determining value.